1. Field of the Invention
The present invention relates to an automatic transmission for an automobile and, more particularly, to an automatic transmission including a gear train composed of a plurality of planetary gear sets.
2. Description of the Prior Art
As is well known in the prior art, the gear trains of an automatic transmission are constructed of a plurality of planetary gear sets of single-pinion type, double-pinion type or Ravignaux type. For the larger number of the planetary gear sets to be used, the number of setable gears is accordingly increased the more to improve the power performance of the vehicle and the initial acceleration, whereas the structure is accordingly complicated to raise the weight and the production cost. For the smaller number of the planetary gear sets to be used, on the contrary, the structure is simplified, although the number of setable gears is decreased, so that the lighter product can be obtained at the lower cost. In the prior art, therefore, various endeavors have been made to satisfy the contradictory requirements for the increase in the number of the gears and for the simplification of the structure. In recent years, there has been proposed a gear train which can set five forward gears and one reverse gear or more gears by using three planetary gear sets.
One example of the proposals is described in U.S. Pat. No. 4,233,861. The gear train is equipped with one Ravignaux type planetary gear set and one single-pinion type planetary gear set. The Ravignaux type planetary gear set has its one pinion gear meshing with a first large-diameter sun gear and a ring gear and its other pinion gear meshing with a second small-diameter sun gear and the former pinion gear, and these gears are carried by a planetary carrier. On the other hand, the ring gear of the Ravignaux type planetary gear set is connected to the ring gear of the single-pinion type planetary gear set. The first sun gear is connected through a first clutch to an input shaft, and the carrier of the Ravignaux type planetary gear set is connected through a second clutch and a one-way clutch to the sun gear of the single-pinion type planetary gear set. There are further provided a third clutch for connecting the second sun gear and the input shaft, and a fourth clutch for connecting the carrier of the Ravignaux type planetary gear set and another input shaft. There are further provided a first brake for braking the rotations of the first sun gear, a second brake for braking the rotations of the sun gear of the single-pinion type planetary gear set, and a third brake for braking the rotations of the carrier of the Ravignaux type planetary gear set.
The automatic transmission described in the U.S. Pat. No. 4,233,861 can set five forward gears and one reverse gear. In order to change the gears from the forward third to fourth, the first clutch and the second brake have to be released, but the second clutch and the fourth clutch have to be engaged. These necessities have raised problems that shocks may occur during a change between those gears and that a delicate gear control is required to suppress the gear changing shocks. In this automatic transmission, moreover, the oil temperature may rise or the durability may drop, because the aforementioned second sun gear is rotated at a high speed for the forward fifth gear and the reverse gear.
The technical disadvantages thus far specified are expected to be eliminated to some extent from the automatic transmission including three planetary gear sets by interposing clutch means between any two of the rotating members of those planetary gear sets so that the rotating members may be connected, if necessary, or disconnected. An automatic transmission equipped with this clutch means is described in U.S. Pat. No. 4,660,439. This automatic transmission has its gear trains constructed of three single-pinion type planetary gear sets. The automatic transmission is equipped, as its clutches for connecting the rotating members of the planetary gear sets selectively, with a clutch for connecting the sun gear of the front planetary gear set and the sun gear of the center planetary gear set, a clutch for connecting the sun gear of the front planetary gear set and the carrier of the center planetary gear set, a clutch for connecting the carrier of the center planetary gear set and the sun gear of the rear planetary gear set, and a clutch for connecting the carrier of the front planetary gear set and the sun gear of the center planetary gear set. With these clutches thus provided, the number of setable gears can be increased while preventing the number of revolutions of any of the rotating members from being increased to a extremely large value. In the case of the gear change from forward second to third or from third to fourth, however, three or more clutches or brakes must have their engaged or released states switched to raise disadvantages that the gear changing shocks occur and that the control becomes difficult. In the structure described in the U.S. Pat. No. 4,660,439, moreover, either the clutch for connecting the sun gear of the front planetary gear set and the carrier of the center planetary gear set or the clutch for connecting the sun gear of the front planetary gear set and the carrier of the center planetary gear set is disposed radially internally of a cylindrical member for connecting the ring gear of the planetary gear set to another member so that its external diameter is restricted. In order to retain the necessary transmission torque capacity, the number of discs has to be increased in accordance with the restrictions upon the external diameter, thus inviting a disadvantage that the total length of the automatic transmission is increased. The pumping of the oil pressure to and from a servo mechanism to engage that clutch is accomplished from the radially inner side because the radially outer side is covered with the rotating member. The rotating shaft extending at the center has to be formed with oil passages for feeding lubricating oil to the planetary gear sets. Even if the oil passages are isolated liquid-tight by means of seal rings, there still remains a problem that the oil passage structures are remarkably complicated. If, moreover, the oil pressure is fed to the servo mechanisms through the inside of the rotating shaft, it is influenced by the centrifugal force established as the shaft rotates, thus raising a problem that the timing for engaging the clutch may fluctuate.
On the other hand, the clutch for connecting the sun gear of the aforementioned front planetary gear set and the carrier of the center planetary gear set may be interposed between another clutch for connecting the sun gears of the front planetary gear set and the center planetary gear set and the sun gear of the center planetary gear set. In the case of this structure, the connecting member between the sun gear of the center planetary gear set and the clutch for connecting the former to the sun gear of the front planetary gear set is elongated so that its weight and accordingly its moment of inertia are augmented. As a result, there arises a problem that the shocks and the durability of the clutches are adversely affected.